I’m not sure why this is a huge issue. Soldiers with military picks can usually be targeted by archers before they even close the distance. Failing that, focus aggression from pole-arms takes out what health they have left. In other cases, a good brother with a shield can avoid/block most hits no problem. Either that or using a character that doesn’t rely on heavy armor reduces the risk that your armor dependent soldiers will be taken out of combat-ready form.
Even if a 300+ piece of armor gets destroyed in a fight, good management makes the problem null. Is your heavy’s armor gone after a nasty fight with a military pick wielding raider? That’s ok, you have replacement armor in your inventory. Pop it on and start repairing the old one before the next fight.